Portsmouth was settled in 1638 by a group of religious dissenters from Boston Colony, including Dr. John Clarke, William Coddington, and Anne Hutchinson. It is named after Portsmouth, Hampshire, England. Roger Williams convinced the settlers that they should go there, instead of settling in New Jersey, where they had first planned on going. It was founded by the signers of the Portsmouth Compact. Its original Indian name was Pocasset. It was officially named Portsmouth on May 12, 1639.

It became part of the colony of Rhode Island (see Aquidneck Island) and eventually of the colony of Rhode Island and Providence Plantations and the state that bears that name.

Portsmouth is the site of one of the earliest and most bizarre murder trials. In 1673 Rebecca Cornell, widow of Thomas Cornell (settler), died in a house fire. Her son Thomas(Jr.) was accused, tried, convicted and hung for the "crime." In addition to there being only circumstantial evidence presented against Thomas, and a number of other potential suspects (including Thomas' wife and a local Native American) and the possibility that Rebecca simply caught fire trying to stay warm near a fireplace, an alleged visit of the ghost of Rebecca to a testifying relative (known as Spectral evidence, which was also admissible evidence in the Salem witch trials) was admitted as solid evidence in the case. This practice of using dreams and apparitions as evidence was subsequently abandoned. This case and its history has been chronicled in the book Killed Strangely: The Death of Rebecca Cornell by Elaine Forman Crane.[3][4]

In 1919, the Hall family started the Quaker Hill Garage, now Aquidneck Construction, Inc., which remains the oldest family-run business in town,[citation needed] specializing in excavation, demolition, and site work. Since 1980, Portsmouth has been home to Clements' Market, a large supermarket. In addition, Portsmouth is home to the Portsmouth Business Park, as well as a few small plazas with a variety of businesses. Portsmouth is also home to a branch of Raytheon, and its Integrated Defense Systems department.

Portsmouth is the headquarters of US Sailing, the National Governing Body of Sailing in the U.S.[5] Portsmouth is also home to the Newport International Polo Series held at Glen Farm.[6] Portsmouth is also home to the Portsmouth Pirates, the town's soccer team. Portsmouth High School also boasts a very successful football and soccer teams. Both teams are regularly in the top 5 teams in the state.

The 2000 U.S. Census[1] reported that there were 17,149 people, or an increase of 1.7%, residing in the town. There were also 6,758 households, and 4,865 families recorded. The population density was 739.0 people per square mile (285.3/km²). There were 7,386 housing units at an average density of 318.3 per square mile (122.9/km²). The racial makeup of the town was 95.82% White, 1.17% African American, 0.19% Native American, 1.36% Asian, 0.03% Pacific Islander, 0.37% from other races, and 1.05% from two or more races. Hispanic or Latino of any race were 1.45% of the population.

There were 6,758 households out of which 33.4% had children under the age of 18 living with them, 61.1% were married couples living together, 8.0% had a female householder with no husband present, and 28.0% were non-families. 23.3% of all households were made up of individuals and 10.0% had someone living alone who was 65 years of age or older. The average household size was 2.53 and the average family size was 3.00.

In the town the population was spread out with 25.2% under the age of 18, 5.0% from 18 to 24, 29.5% from 25 to 44, 26.9% from 45 to 64, and 13.5% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 96.5 males. For every 100 females age 18 and over, there were 92.6 males.

The median income for a household in the town was $58,835, and the median income for a family was $68,577. Males had a median income of $46,297 versus $31,745 for females. The per capita income for the town was $28,161. About 2.0% of families and 3.4% of the population were below the poverty line, including 2.8% of those under age 18 and 6.4% of those age 65 or over.

The 2010 U.S. Census[1] reported that there were 17,349 people, or an increase of 1.15%, residing in the town. The racial makeup of the town was 94.57% White, 1.35% African American, 1.58% Asian, 0.21% American Indian or Alaskan Native, 0.04% Native Hawaiian or Pacific Islander, 0.40% of some other race, and 1.86% of two or more races.

In the town, 22.98% of the population was under the age of 18 and 16.47% were 65 years of age or older. Females made up 51.03% of the population.

The 1725 schoolhouse owned by the Portsmouth Historical Society is one of the oldest surviving in the U.S.